1. Field of the Invention
The present invention relates to telephone systems and, more particularly, to a method and apparatus for a private branch exchange (PBX) to sense external alarms using standard telephony interfaces.
2. Description of the Related Art
Private branch exchanges (PBX's) are circuit switches that provide communication paths among a plurality of users and communications systems, typically within the limits of a private institution. A PBX connects the users to one another, as well as interfaces to the public switched telephone network (PSTN), for example, via a local central office. In addition, a PBX can interface telephony conversations, voice or phone mail, facsimile, and electronic mail systems. In addition, the PBX may provide a variety of customer features, such as call hold, call forwarding, automatic call-back, conference calling, and the like.
Thus, PBX's are required to monitor a variety of trunk and telephony lines for a variety of conditions. For example, in the case of a telephone connection, a PBX monitors the line for on- and off-hook states. In response to detecting an off-hook state, the PBX supplies a dialtone and waits to receive dialed digits (either pulse or DTMF (dual tone multi-frequency) tones). The PBX further identifies the call destination, sends a ringing signal via the appropriate destination line or trunk, and either provides a busy signal or connects the subscriber with the called party. The PBX also provides a fast busy signal if no circuits are available.
The PBX may also provide a variety of maintenance features. Thus, the PBX monitors the telephone lines for conditions indicative of line faults, such as by detecting changes in impedance along the lines. For example, in the case of a subscriber telephone unit, the PBX can detect an open-circuited conductor, a short circuit between tip and ring conductors, short circuits to ground, or undesired electrical paths between subscriber loops. Another type of fault can occur if the off-hook state is detected and no dialed digits are detected; in response, the PBX transmits a warning signal across the telephone line to alert the subscriber that the telephone is off-hook.
In order to accomplish this functionality, PBX's include a plurality of input/output circuits, or ports. These enable the PBX to connect, for example, via telephone, data and trunk lines to the appropriate connected system or systems. In addition, PBX's typically include a predetermined number of alarm ports. The alarm ports are dedicated ports configured to receive alarms signals from a particular device. Such alarms can include, for example, indications of system failure from linked telephony systems (i.e., other PBX's, cellular telephone base stations, voice mail, facsimile and e-mail units), indications of individual device failure, or alarms related to premises activity and unrelated to specific telephone or telecommunications activity, such as intruder or fire alarms.
Alarms are activated at the alarm device and sensed at the PBX through standard circuitry. More particularly, alarms are activated at the alarm device through a circuit known as an alarm contact closure point. The alarm contact closure point is essentially a switch that is closed upon the sensing of the associated alarm condition. In an analog telephone system, activating the alarm contact closure point causes a change in impedance of the line connecting the alarm contact closure point with the PBX. Typically, a circuit is completed, and current flows across the line. The alarm port at the PBX detects the current or change in impedance and, for example, signals an interrupt executable by a control processor at the PBX. In a digital telephone system, activation of an alarm contact closure point causes a predetermined signal to be returned to the PBX, which is detected as the alarm condition.
The number of alarm ports provided on PBX's is quite small (typically fewer than three are provided), due in large part to space constraints. As the number of alarm ports increases, the number of ports available for data and telephone and trunk lines necessarily decreases correspondingly. However, the number of systems which are typically desired to be connected to PBX's and to provide alarm indications has increased.
Accordingly, in many systems, external equipment such as an external switching device or multiplexer is provided to receive a plurality of alarm lines and multiplex them to the PBX's alarm port. Because the alarm port on the PBX is a standard and relatively unsophisticated interface, however, while the PBX receives the alarm, it is unable to identify the particular device which has triggered the alarm. Thus, the external switching device must be provided with additional circuitry to identify which device has triggered the alarm and, also, in many cases, to provide a modem for providing an alarm message to an appropriate service bureau or to the PBX itself.
Such external equipment is not necessarily provided by the same vendor as the vendor of the PBX itself, and as such, additional logistical and training costs are incurred. Therefore, an improved alarm sensing mechanism is desired. Nonetheless, the number of existing alarm devices is quite large and any alteration to the accepted standard would encounter user resistance unless backwards compatibility were provided. Therefore, an improved alarm sensing mechanism and method is desired which minimizes external equipment and maintains backward compatibility with prior alarm devices.